A compact and thin-type image pickup apparatus has come to be mounted on a mobile terminal representing a compact and thin-type electronic hardware such as a cell-phone and PDA (Personal Digital Assistant), whereby, it has become possible to transmit mutually not only voice information but also image information to a remote location.
As image pickup elements used for these image pickup apparatuses, a solid-state imaging device such as an image sensor of a CCD type and an image sensor of a CMOS type are used. In recent years, increase of pixel numbers of the image pickup element have been advanced, and enhancement of its resolution and performance have been attained. As a lens for forming an image of a photographic object on the imaging device, a lens made by resin that is suitable for mass production has come to be used for further cost reduction. In addition, the lens made by resin is excellent in terms of its workability and has satisfied requirements for enhancement of its performance, by being formed in aspheric shape because its workability is excellent.
As an image pickup lens of this kind used for an image pickup apparatus housed in a mobile terminal, an optical system of a three-element structure including three plastic lenses and an optical system of a three-element structure including one glass lens and two plastic lenses are widely known. However, a demand for further compactness for these image pickup lenses and a demand for mass productivity required for the mobile terminal are becoming stronger, and the compatibility between both demands is becoming more difficult.
To solve these problems, there has been proposed a method to produce a large number of lens modules as followings. There is provided a glass substrate in a size of several inches, which is formed in a parallel flat plate. A large amount of lens elements are simultaneously formed on the glass substrate through a replica method. Then, the glass substrate (lens wafer) on which a large number of lens elements are formed is combined with a sensor wafer and is cut off to produce a large number of lens modules. Lenses manufactured by this method are called wafer-scale lenses, and lens modules manufactured by this method are called wafer-scale lens modules.
Additionally to the method to produce a large number of lens modules, there has recently been suggested a method to mount the large number of lens modules on a substrate at low cost, as followings. Lens modules are arranged together with IC (Integrated Circuit) chips and other electronic parts on a substrate on which a solder is potted in advance. By adding reflow processing (heating processing) to the substrate to melt the solder as the lens modules are arranged thereon, the electronic components and the lens modules are simultaneously mounted on the substrate. Image pickup lenses that withstand reflow processing and are excellent in heat resistance is also demanded.
As the image pickup lens of this kind, there are proposed lenses shown in JP-B Nos. 3929479 and 3976781 which disclose lens blocks of a two-element structure. However, in these lenses, aberration correcting power is insufficient, and it is difficult to say that these image pickup lenses sufficiently copes with a solid-state imaging device with larger number of pixels. In particular, chromatic aberration is hardly corrected in these lenses, thereby, an image pickup lens shown in JP-A No. 2006-323365 in which a diffractive surface is applied on a lens substrate is also proposed.
However, in the image pickup lens shown in JP-A No. 2006-323365, the application of the diffraction surface rises the degree of difficulty in manufacturing, and decreases diffraction efficiency for a wavelength other than a design wavelength. It generates a diffracted light of unwanted order, resulting in a problem of ghost.